Ed Balls made a statement in the House of Commons on the Sats marking fiasco, but new information has since come to light. Photograph: PA

It was the biggest administrative failure to strike British education in recent years. It led to 1.2 million pupils' test results being delivered late, delays to school league tables and the eventual scrapping of national testing in secondary schools. But who should really shoulder the blame for last year's national curriculum test marking fiasco?

An official inquiry last December appeared to have laid this question to rest. Responsibility largely fell on the American marking contractor, Educational Testing Services (ETS), the Qualifications and Curriculum Authority (QCA) and its subsidiary, the National Assessment Agency (NAA). Ken Boston, QCA chief executive, and David Gee, the NAA's managing director, have since resigned, while ETS had its marking contract terminated. The government itself largely escaped blame.

But new documents have come to light that will place fresh pressure on ministers and their officials, who were being kept informed in detail of the problems as they developed. The papers show that Department for Children, Schools and Families (DCSF) officials joined those from the QCA and the NAA in reassuring ministers that the situation was in hand, even as scores of markers complained in online discussion forums that the process was chaotic.

Yet, despite these assurances, the documents suggest that ministers were still being given enough information to conclude that things were going badly wrong with the tests months in advance of results being issued. In fact, Ed Balls, the schools secretary, was told of risks to the delivery of the national tests in early February last year, five months before the government eventually had to admit that results would not be released on time. At the least, the documents may suggest that accountability was blurred.

The documents also raise questions as to why the content of these DCSF briefings for ministers was not mentioned in the independent Sutherland inquiry report.

Meanwhile, a recent report by the Office of Government Commerce (OGC), part of the Treasury, suggests it believes that the DCSF bore a key responsibility in approving the appointment of ETS as the test marking contractor.

The marking of the key stage 2 and 3 tests taken by 1.2 million pupils in May 2008 has been described by Balls as a "shambles". From examiners being sent contracts to mark the wrong subjects to software systems crashing and scripts being discovered unmarked, the organisation was chaotic.

Eventually, such results as were available were released to schools a week late, in mid-July. Many had to wait until the autumn term for their pupils' marks.

In July 2008, Ofqual (Office of the Qualifications and Examinations Regulator) and the DCSF commissioned Lord Sutherland of Houndwood, a former chief inspector of schools, to launch an inquiry. He reported in December that primary responsibility lay with ETS, but that the QCA and NAA had also failed.

Speaking in the Commons, Balls then rejected claims from opposition MPs that ministers were shrugging off responsibility. Referring to the Sutherland report, Balls said: "The QCA and NAA at executive level failed to escalate risks upwards to the board and to ministers, who were given repeated assurances that things were OK when they were not. Those executives then failed to follow up problems when they were identified." He added: "Time and again during 2008, my officials and ministers raised questions with the QCA and were reassured that things were OK." It was only on 30 June last year, 10 days before the results were due to be published, Sutherland found, that the QCA told ministers they would be delayed. Balls has also said that the QCA managed its contract with ETS "at arm's length" from ministers.

Some 49 papers, including minutes of meetings and emails between officials, are now accessible on the DCSF website.

In a four-page briefing in April 2008 sent to Balls – and with the ministers Jim Knight and Andrew Adonis copied in – Sue Hackman, the government's chief adviser on school standards, writes of an earlier letter the DCSF had written to Balls on 8 February "alerting him to a potential risk to the delivery of this summer's national curriculum tests".

This related to a trial in autumn 2007 by ETS of a new system of online training for markers, which had not gone well. Hackman writes in April that the trial "did not provide any robust evidence of improvement in marking quality."

She then sets out a series of risks to the delivery of national tests, including "the lack of experience of the new contractor in UK test delivery and the weakness of their management capability".

Hackman then says: "Although these risks are significant, we are content with NAA's contingency arrangements, and mechanisms for monitoring them."

A further memo, dated 15 May – only days after pupils had taken the first of the tests – from an official in the DCSF's assessment team to Balls and Knight, was sent to warn them in anticipation of a newspaper report that would say the marking process was already in disarray.

The memo warns ministers of a series of problems, including ETS's helpline for markers receiving 10,000 calls in a single day and a faulty IT system temporarily wrongly barring 40% of all KS3 English markers from their work.

However, the DCSF memo concludes: "We are continuing to monitor the situation and are confident that NAA has the situation under control, and is actively supporting the new delivery contractor, ETS Europe, to secure test delivery."

An update to ministers from officials dated 10 June and referring to a briefing note from the NAA says: "The NAA says that they are now confident that they can deliver results to schools by the scheduled deadline of 8 July." However, the NAA note itself says only that it will be "possible" to meet this deadline.

None of these briefings for ministers by officials is mentioned in detail in the Sutherland report, or listed in its reference sources. Neither are the department's verdicts on risk mitigation at the NAA.

In his speech to Parliament, Balls quoted the Sutherland report's conclusion that ministers were told that results would definitely be delayed only on 30 June, 10 days before they were due. This is true. But it is clear that, by mid-June, they were aware of the risk that some results would not be released.

An update on the marking situation dated 19 June, which appears to have been written by DCSF officials, says: "NAA is confident that these arrangements will deliver results on time ... but we believe that there is still a risk that KS3, in particular, may be delayed."

During April, May and June, in a series of "status reports" to the DCSF on marking progress, the NAA undoubtedly appears to play down problems, saying repeatedly that results can be delivered on time. It also admits to a string of difficulties, however, including that delivery of scripts to markers was "slower than expected".

Then, on 30 June, Gee confesses that many results will not be back by the 8 July deadline. In an email to DCSF officials, he provides a list of 36 "interventions" that the NAA had taken with ETS management to try to avert the failure, including "direct intervention by NAA programme director to underpin weak ETS leadership team".

The documents also include an NAA admission that, in the English tests "consistent application of the mark scheme by markers is difficult". And, in an email to Boston on 16 June, Gee says: "I trust DCSF will not brief against me in the meeting I have with Jim Knight tomorrow."

Nick Gibb, shadow schools minister, says: "Contrary to the assertion by the secretary of state that he and his department were not involved, these documents make it very clear that they were informed in great detail about the risks with the management of the ETS contract. Why are these memos, which have now appeared in great detail on the DCSF website, not included in the list of documents quoted by Sutherland?

"Given the number of DCSF discussions between February and May 2008 about these issues, and the absence of significant action by ministers in that period, the only conclusion you can draw is one of huge complacency by ministers. Why did they not act?"

Lord Sutherland tells Education Guardian: "I had access to any documents which I wished to see. There were no restrictions." He adds that he did have concerns in relation to some DCSF functions, specifically the "unclear" role of DCSF officials as observers on committees of organisations such as the QCA.

He says: "The job of an 'arm's length' body is to deliver. The job of the department on behalf of ministers is to probe and where necessary to ask questions and seek reassurances."

The OGC report includes a letter sent in January from Nigel Smith, its chief executive, to David Bell, the DCSF's permanent secretary. It suggests that the OGC believed the department bore some responsibility for the process by which ETS was originally appointed. The Conservatives have argued there should have been more checks on the company.

A DCSF spokesman says: "There is nothing new in these documents. The documents show DCSF officials challenging the QCA/NAA on the apparent problems in receiving robust reassurances. Lord Sutherland has considered the evidence and reached the conclusions he sets out in his report: that the prime responsibility lay with ETS but that there were also significant failings in QCA. He criticised DCSF for not making clearer the role of observers in the process and we have rectified this.

"Lord Sutherland does in fact refer to these documents. For example, he writes: 'officials updated ministers on risks throughout the test cycle and provided almost daily written briefings during June 2008'."

The spokesman adds: "The OGC has revised its mechanisms for assessing risk and the department has also taken steps to improve its own internal assurance mechanisms."

The OGC letter says that two sets of checks on the procurement process that was used to award ETS the marking contract in 2007 were led by the DCSF. Both, in December 2006 and September 2007, assessed the project as "low risk". OGC had itself carried out an earlier check, which said it was "medium risk".

An annex to the OGC report concludes: "The project should have been regarded as high risk from the beginning."